Every software package contains a full set of examples suitable for that version and are installed with the software. If you see examples here that are not in your installation you should consider updating to a later version of the software. These examples can be used by beginners and advanced users for product evaluation, teaching purposes, and self-training.
Required Modules
Victory Device
- 2D Simulator
This example demonstrates simulation of avalanche breakdown in a single quantum-well lattice matched AlGaAs/GaAs HEMT. It shows:
Construction of the heterojunction structure using Victory Device syntax
Material and models parameter specification
Simulation of Id-Vds and breakdown characteristic
Display of the results in Victory Visual
The device under consideration consists of the highly doped (1.e18/cm3) AlGaAs layer with the Al composition fraction of 0.3, the GaAs channel layer 250 A thick with the donor concentration of 1.e15 /cm3, and AlGaAs buffer layer with the same doping. The parts of the channel under the source and drain are heavily doped, and source and drain electrodes are located vertically, touching the channel region. This technique accounts for the alloying of contact material and the tunnelling of carriers through the heterojunction. The channel length is 0.5 micron.
In the first part of the input file, the device is described using the Victory Device structural syntax. The description includes the mesh, regions locations, electrodes locations, and doping distribution. The region statements are used to define the AlGaAs and GaAs regions. The Al composition fraction (x.composition=0.3) is defined here as well. Note that an artificial oxide layer (region number=4) is defined here providing the possibility to specify AlGaAs surface states using the interface statement.
After the device description, the first material statement is used to specify the electron and hole SRH lifetimes applied to both GaAs and AlGaAs materials. In the second material statement the low field mobilities, electron affinity, and density of states in the conduction band Nc are defined for AlGaAs. In this example the band alignment is calculated by using energy band gaps and electron affinities of the materials forming heterojunctions. If not explicitly specified in the material statements the default values of other material parameters are applied.
The models statements are used to specify the following set of models : field dependent mobility, and SRH. In addition, the concentration dependent mobility model is activated.
In order to simulate avalanche breakdown the impact ionization-generation model should be turned on. This is done using the impact selb statement in which the Selberherr impact ionization model is activated. All basic impact ionization parameters are user-accessible and can be modified by the user. As an example, the appropriate set of parameters is defined in this statement. The parameters apply to both GaAs and AlGaAs materials.
The gate electrode in HEMT structures is of Schottky type. To indicate this the contact statement is used to define the workfunction of the gate electrode, which gives the Schottky barrier height of approximately 0.9V.
The initial solution is performed automatically if not specified. The gate voltage is set to zero and the drain voltage is ramped up to 0.4V.
Next the drain voltage is ramped until the drain current reaches the predefined value of 1.e-4 A/um well within the breakdown region, using the compliance limits on the solve statement. Before moving to current boundary conditions for the final ramp to breakdown.
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